Did Climate Change Kill the Mayans?

There are a lot of things that didn’t kill the Mayans: asteroid strikes, planet-wide quakes, global cataclysms prophesied by shamans and etched into ancient calendars. What did wipe them out was likely something that is far less mystical, and indeed is entirely familiar to modern civilizations: climate change. If you want a look at what we could face in the decades and centuries ahead, look at what one of the world’s greatest cultures suffered a millennium ago. That’s the conclusion of a newly released study and what it lacks in Hollywood-friendly drama, it makes up in sound — and scary — science.

The arc of the Mayan rise and fall is well known: The civilization first took hold in 1,800 BC, in the Central American region that now includes and surrounds Guatemala. It grew slowly until about 250 A.D. At that point, a great expansion of the culture — known to archaeologists as the Classic Period — began and continued to 900 A.D., yielding the architectural, political and textual artifacts that have so mesmerized scientists. But a decline began around 800 A.D. and led to a final collapse about 300 years later.

The Mayan arc was hardly smooth and steady, and there were periods of turbulence and decline even during the golden era. The great settlement of El Mirador, which once might have been home to 100,000 people, collapsed around 300 A.D, for example. From the fifth to eights centuries A.D., there was an explosion of the rich tablet texts that provided so many insights into how the Mayans lived and worked. Suddenly, however, starting in 775 A.D., the number of texts began to plunge by as much as 50%, a bellwether of a culture that was declining too.

There have been a lot of theories for what accounted for such cycles, with climate among the most-mentioned. The better the year-to-year weather — with plenty of rainfall and reasonably steady and predictable temperatures — the better crops do, and the more the culture and economy can expand. The texts have hinted at declines in productivity, perhaps climate-related, coinciding with generations of unrest, but there was never a precise way to confirm those writings. Analysis of lake sediments can yield a reliable reading of the levels of sulfur, oxygen isotopes and other atmospheric markers at various points in history, which reveal a lot about rainfall and other critical variables. But the Mayans themselves often unwittingly disturbed those sediments, with deforestation — including wide-scale burnings — and fishing.

Anthropologist Douglas Kennett of Penn State University, leader of an international team of researchers from the U.S., Belize, Switzerland, Germany and elsewhere thus decided to look at another, less vulnerable, source of evidence: stalagmites in caves. Some of the rainfall absorbed by the ground over the course of centuries will seep into caves and be incorporated into the drip-drip-drip of wet limestone that causes stalagmites to form. Oxygen isotopes entrained in the rain can provide an indicator of how wet a region was at any one point in history.

As Kennett and his colleagues reported in the current issue of Science, they focused their investigation on a cave in the jungles of Belize, which is within 1.5 km (.9 mi.) of one significant Mayan site and 19 mi. (30 km) of three others. In 2006, the scientists harvested a 22-in. (56-cm) stalagmite from deep within the cave. Knowing the rate at which stalagmites develop, they could calculate that the top 16.3 in. (415 mm) of it had been growing continuously since 40 B.C. Every 0.1 millimeter — or about four one-thousandths of an inch — corresponded to about 0.5 years. That’s an awfully fine-grained way to look at history, and the analysis led to some awfully detailed conclusions.

Droughts lasting at least a few decades each occurred from 200 to 1100 AD, and repeatedly coincided with struggles and upheaval in the Mayan culture. There were dry periods in the 640 to 660 window, for example, and there was also a lot of warfare in that period, which makes sense for a culture fighting over swindling resources. Droughts from 820 to 870 similarly were associated with an outbreak of fighting, as well as the disintegration of local polities, or ruling bodies. The decline in historical texts began not long before another period of severe drying, and the collapse of the Mayan culture itself, directly corresponds to the most severe period of drying, from 1020 to 1100.

Broadly, explains Kennett, the most generous period of rainfall during the millennium or so the Mayans thrived was from 450 to 660. “This led to the proliferation of cities like Tikal, Copan and Carasol,” he says. “The new climate data show that this salubrious period was followed by a general drying trend…that triggered a decline in agricultural productivity and contributed to social fragmentation and political collapse.”

The cause of the climate upheaval that claimed the Mayans was not, of course, human activity, since their culture thrived and died long before the industrial age. Instead it was caused by the combined effects of El Niño events and changes in the northeast and southeast equatorial winds known as the intertropical convergence zone. “The preceding conditions stimulating societal complexity and population expansion helped set the stage for later stress on [Mayan] societies and the fragmentation of political institutions.”

This should not give comfort to the dwindling band of modern-day climate-change deniers likely to take the study as proof that the planet’s current climate woes are merely natural fluctuations. The Mayan drying trend played out over centuries, after all. What we’ve been experiencing is the more abrupt, short time-scale variety, a phenomenon that was first foreseen in climate models nearly 40 years ago, and has been unfolding pretty much on schedule and as predicted. The Mayans had no such power to forecast events and certainly couldn’t correct them. We can do both, and the lesson of the study is that we may pay mightily if we choose to take no action. No culture, as the vanished Mayans so starkly illustrate, is too big to fail.